Investigation of the Localized Corrosion of Ni-Cr-Mo-W Alloy UNS N06022 in Aprotic Solvents at Room Temperature

Syntheses of pharmaceuticals often require a series of intermediate steps involving reactions in aprotic organic solvents. Some of these solutions can be aggressive towards the metallic materials that constitute the reaction vessels used, so often corrosion-resistant alloys (CRA) such as the Ni-Cr-Mo-W UNS N06022 are employed. Such materials are expensive, but their use is required to prevent contamination of the product by metal ions. Ni-Cr-Mo-W alloys are known to exhibit outstanding corrosion resistance in reducing and oxidizing aqueous solutions as a result of their passive oxide film. However, the application of passive alloys in nonaqueous solvents raises additional concerns due to: i) limited availability of oxygen atoms for passivation; ii) different behavior of aggressive anions and acids as a result of the solvent’s physicochemical properties. Nevertheless, the literature on corrosion of CRA is focused on behavior in aqueous solutions and the literature on corrosion in organic solvents lacks information on the application of commercial alloys. The aim of this work is to investigate the corrosion of Ni-Cr-Mo-W alloys in aprotic organic solvents and to understand the influence of properties of the solvent on the susceptibility to pitting and to uniform corrosion, at room temperature. Preliminary work was carried out with anhydrous acetonitrile solutions containing HCl and the oxidant 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ). Mass loss and electrochemical tests have shown that alloy N06022 exhibits high-rate dissolution in such conditions. The corrosion rates observed after 24 h of immersion are greater than 10 mm/year, which is remarkably high when compared with the existing literature on corrosion of Ni-Cr-Mo alloys. Moreover, in neutral acetonitrile containing chloride, N06022 undergoes pitting at much lower Cl– concentrations and temperature than in aqueous solutions. In order to understand the enhanced susceptibility to pitting and uniform corrosion in aprotic solvents, potentiodynamic polarizations and electrochemical impedance spectroscopy were performed in various solution compositions, followed by surface characterization with scanning electron microscopy. The correlation of passivation and breakdown phenomena with variables of importance, namely water content, chloride concentration, and acidity will be investigated. Ultimately, this study will provide fundamental understanding of the passivity of Ni-based alloys in organic solvents, assisting the selection of materials for nonaqueous applications.